In heating systems for residential and commercial buildings, the thermal power requirement is typically a function of the external environmental temperature. The thermal load of the building becomes greater as the external temperature falls.
The power of the heating appliances for a given building is chosen for a design temperature (defined by regulations for every given place) which is normally close to the minimum mean temperature recorded over a long span of years. In north Italy this is often between −10° C. and −5° C., whereas in central Europe it is often at −20° C.
The temperature of the hot water of hydronic systems (common in the whole of Europe and north-east USA) is a function of the external temperature, according to a selectable “climatic curve” which enables better regulation of the power delivered by heating systems, and hence better comfort and less utilities consumption. This curve provides higher hot water plant delivery temperatures for lower external temperatures. A typical example is 70° C. delivery at −20° C., with 100% system power, 40° C. at +7° C. external temperature, with 35% system power. The operating conditions at low temperature do not have a determining impact on the system energy efficiency profile, because in the whole of Europe they affect only a small fraction of the total energy consumed for heating (which is delivered at mean ambient temperatures between 0° C. and 2° C.); in contrast, they have a determined impact in determining the maximum power and the temperature of the system water to be supplied.
In the current search for reducing the environmental impact of such activities, and in particular of heating (and domestic hot water), which in Europe is mainly responsible (equally with the transport sector-source CE/Ecoboiler study) for CO2 emissions, the European Commission has composed a scale of the most efficient heating systems currently existing. The first place is occupied by heat pumps. It is evident that heat pumps, both electrical and absorption (GAHP), are receiving considerable interest. However, the application of heat pumps for heating purposes encounters certain obstacles which slow down their distribution. In particular, the profile of delivered power and temperatures obtainable does not follow that typically required by buildings. For example, heat pumps have efficiencies even double those of a boiler at temperatures of 7° C. and 40° C. for 100% of available power, but difficultly reach a delivery of 70° C. (currently only absorption heat pumps reach this figure), and often with powers equal to 30% for electric pumps and 60% for absorption pumps. Hence if sized to operate at the point of major energy interest, i.e. for an ambient temperature of about 2° C., heat pumps deliver a power and water system temperatures which are too low at design conditions.
In the known art this situation compels the following applicational solutions:
A first solution consists of over-dimensioning the heat pump to be installed. In addition to the greater cost, the system is considerably oversized under typical conditions, with working conditions which are penalizing because of the operation at very low partial loads (such heat pumps sometimes work for several hours at loads of 15%).
A second solution consists of adding a back-up system, typically an additional boiler or a set of electrical resistance elements (many kW of power required). All this makes the application of the heat pump even more costly, but above all more complicated to control (electronic integration system between the systems, and maintenance).
Moreover the temperature of the hydronic system water required in older homes (90% of all homes) is often >70° C., and up to 80° C. under very cold external ambient conditions. In such conditions none of the current heat pumps is able to satisfy these requirements.
To confront the aforestated problems, absorption heat pumps have been constructed composed not only of the thermodynamic circuit but also of a combustion system and a heat exchanger (generator) to which the combustion system provides the combusted power. In present-day absorption heat pumps, this combustion system is modulating and similar to that used on common boilers. An absorption heat pump requiring for example 12 kW of rated power can be served by a combustion system delivering from 20 to 3.5 kW.
It can therefore be considered that if greater power and delivered water temperatures are required (such as during low outside temperature), it would be sufficient to increase the input to the generator. In reality, under these conditions the effect of the increase in thermal input is a continuous increase in generator temperatures until one or other safety member intervenes.
U.S. Pat. No. 4,364,240 (expired) FIG. 3 shows how to increment the output delivered by a heat pump under conditions in which the evaporator no longer manages to recover heat from the low temperature source (e.g. low external temperature conditions). For this purpose a bypass for the rich solution is inserted, to feed this directly into the generator by bypassing the recovery heat exchangers within the cycle.
The bypass for the rich solution does not by itself enable the generator input to be increased sufficiently to force the delivery water temperature to the required levels under very low external temperature conditions. EP1233240-A2 and EP0001858-A1 describe absorption heat pumps pertaining to the known art.